ENZYMES
Definition
• These are organic substance that
accelerates the rate of chemical reactions.
• Organic catalyst are enzyme:
• 1.Highly specific: catalyze one or two
reactions only.
• 2.Protein in nature: denatured by heat.
• Inorganic catalyst are metals:
• 1.Non-specifin:catalize many reactions
• 2. Not affected by heat.
the molecules at the
beginning of the process
are called substrates, and
the enzyme converts them
into different molecules,
the products.
Enzymes
• Have a recommended name
• Suffix –”ase” attached to the substrate of
the reaction e.g. glucosidase,
”ambreenase”, “vishaalase”
• OR to describe the action performed. e.g.
lactate dehydrogenase
4-types of specificity
• Optical specificity: acts on one of 2
isomers. e.g maltase acts on αglycosidase and not β-glycosidase.
• Group specificity: the presence of certain
group.e.g pepsin acts on peptide bonds.
• Absolute specificity: only on one
substrate e.g urease acts only on urea.
• Relative specificity: acts on group of
compound having same type of
bonds.lipase acts on different triglycerides.
Enzyme activity
•
•
•
•
Enzymes : simple or conjugated.
Simple: native conformation of protein.
Conjugated protein: holoenzyme.
Apoenzyme + cofactor-> holoenzyme.
..truly amazing substances
• Active sites: special pocket where
substrate binds (reaction occurs)
• Catalyze reaction 10 3 TO 10 17 TIMES
FASTER than uncatalyzed reactions
• Are highly specific i.e. can catalyze only
one type of reaction
• Some enzymes require an additional
chemical component-COFACTOR e.g.
metal ions such as Zn 2+, Fe 2+, Mg 2+
..still on properties
• or……….an organic molecule called a
coenzyme e.g. NAD+ contains Niacin,
FAD contains riboflavin
• Holoenzyme - enzyme with its cofactor
• Apoenzyme -protein portion of the
holoenzyme.
• Apoenzyme shows no biologic activity
without appropriate cofactor
Zymogens
• Some enzyme are synthesized as inactive
forms called zymogen or proenzyme e.g
trypsinogen and pepsinogen.
• 1. Zymogen are inactive because their
catalytic sites are masked by a polypetide
chain.
• 2. To activate, cleavage of polypeptide
chain.
IUBMB
Systematic name
Enzymes are divided into 6 major classes
Suffix “–ase” is attached to describe the
chemical reaction catalyzed
1.Oxidoreductases
2.Transferases
3.Hydrolases
4.Lyases
5.Isomerases
6.Ligases
Oxidoreductases
• Enzymes catalyzes an oxidation-reduction
reaction between two substrate.
• S(oxidised)+Y(reduced) s(reduced) + Y
(oxidised).
Transferases
• Enzyme catalyzes the transfer of a group
other than hydrogen from one substrate to
another .
• SX + Y  S + YX
• Glucose +ATP------glucokinase--------------------- gucose-6-phosphate +ADP.
Hydrolases
• Catalyzes hydrolysis.
• A –B --HOH---- AH +BOH
• peptidase
Lysases
• Catalyzes removal of group from substrate
by mechanism other than hydrolysis.
• Decarboxylase.
• Pyruvate acetladehyde + CO2
Isomerases
Transfer of groups within molecules to yield
isomeric forms
one substrate and one product are involved,
Ligases
Formation of C-C,
C-S,C-O and C-N bonds coupled to hydrolysis of
high energy phosphates e.g. ATP
Often referred to as SYNTHETASE
LOCK AND KEY HYPOTHESIS
Induced fit theory
Mechanism of enzyme action
• Free energy of the reaction: initial state to
final state , consume energy.
• Activation energy: absorb energy
(activated state or transition state).
• The effect of enzyme is to decrease the
energy of activation.
More properties
• Enzyme activity can be regulated i.e.
activated or inhibited so rate of product
formation suits demands of the cell
• Are reusable because they aren't altered
in a reaction
Factors affecting reaction
velocity
•Substrate
concentration,enzyme
concentration.
•Temperature
•pH
Concentration of enzyme
• The amount of enzyme is in a reaction is
doubled ,the amount of substrate is
converted to product is doubled.
Concentration of substrate
• At low substrate concentration ,not all
enzyme are saturated .So the rate of
reaction will increase.(V max)
• At higher substrate concentration ,all
enzyme molecules get saturated with
substrate and any more increase of
substrate concentration will result in no
increase.
Michaelis –menten equation
• It describes the dependence of reaction
velocity on substrate concentration.
• E + S –k1ES
• ES breaks down to give enzyme and
product.
• E + S K-1
E+ S
•
•
K2
E
+P
So, K1 , K-1,and k2(rate constants)
Cont.
• Vi=Initial velocity
• Vmax =all enzyme are involved in an ES
complex.
• [s]= increased Substrate
• Vi=Vmax [s]
[s] + (k1+k2)
K-1
Cont.
• The ratio constants k1 + k2/k-1 as
michaelis constant(k ) .
• So km=k1+ k2
k-1
then vi = Vmax [s]
[s] + km
Michael equation=km
When substrate conc.[s] is equal to km
Vi =vmax[s] = vmax [s] =Vmax
[s] + km
2S
2
m
Cont.
• Thus km can be defined as substrate that
produce half maximum velocity.
Effect of temperature
• The optimal temp. for enzyme activity in
human body is that temp similar of that the
cells(37˚)
• At zero,enzyme inactive
• The velocity is almost doubled every 10˚C
• At 55˚-60˚C ,most enzyme are denatured
and become permanenlty inactive.
Effect of pH
• The optimal pH activity is that pH at which
the enzyme acts maximaly.
• Above and below, the reaction decline.
• Each enzyme has has its own optimal
pH.e.g pancreatic lipase 7.5
• Extream of pH leads to denaturation.
Enzyme activators
• To activate the enzyme
• Metal ions;
• Chloride ions which activates salivary
amylase and calcium ions which activate
blood clotting enzymes.
• Zymogen needs other enzyme for
activation
Inhibition of enzyme activity
• Any substance that can diminish the velocity of
an enzyme-catalyzed reaction-INHIBITOR
• Is Reversible (through non-covalent bonds) or
Non-reversible (through covalent bonds)
TYPES
• Competitive: reversible binding to the same site
for the substrate and competes actively for the
site.
Methotrexate
A competitive inhibitor of dihydrofolate reductase - role in purine
& pyrimidine biosynthesis
Used to treat cancer
40
• statin drugs competitively inhibit 1st
committed step in cholesterol synthesis
catalyzed by HMG CoA Reductase e.g.
latorvastatin (Lipitor)-↓ plasma cholesterol
levels.
• Non-competitive: inhibitor and substrate
bind @ diff sites. either free enzyme or ES
complex and prevents reaction from
running e.g. lead inhibiting ferrochelatase
• Enzyme inhibitors can be used as drugs
e.g. Penicillin, amoxicillin, ACE inhibitors
Allosteric regulation of enzyme
activity
• allosteric enzyme generally catalyze the
irreversible steps in metabolic pathways.
• Allosteric mean “other site”,they bind noncovalently at a sit other than active site.